Method and Device for Measuring the Magnetic Properties of Documents

ABSTRACT

The present invention relates to a method and an apparatus for measuring magnetic properties of a document ( 5 ) and to a measuring head ( 12 ) suitable therefor for measuring magnetic field changes. The apparatus comprises a device ( 2, 3 ) for generating an electromagnetic alternating field, a measuring element ( 6 ) and a lock-in amplifier ( 7 ). The measuring element ( 6 ) is so adapted that it converts an electrical input signal of the measuring element ( 6 ) into an electrical output signal in dependence on changes of the magnetic field when the document ( 5 ) with magnetic properties is brought into the magnetic field. The measuring element ( 6 ) used is preferably as a GMR or SDT element which changes its electrical resistance comparatively strongly upon even small changes of the electromagnetic alternating field. The measuring head ( 12 ) for use in the apparatus comprises a printed circuit board ( 13 ) with coils ( 14 ) disposed thereon and a GMR or SDT element ( 6 ).

The present invention relates to a method and an apparatus for measuringmagnetic properties of documents, in particular bank notes, and to ameasuring head suitable therefor for measuring magnetic field changes.

Methods and apparatuses for measuring magnetic properties of documentsare known in which a magnetic field is generated by means of a permanentmagnet. In this regard, DE 40 22 739 A1 describes an apparatus with amagnetic circuit, consisting of soft magnetic and permanent magneticmaterial, the static magnetic field generated by the permanent magneticmaterial penetrating the magnetic circuit. The magnetic circuitgenerates a stray field which undergoes changes when a test object withmagnetic particles is moved into the stray field area. Said changes aredetected by means of a coil by a voltage being induced in the coil dueto the changes. With this measuring principle, the change to be measuredcan be noticeably influenced by even small external influences, whichadditionally impedes the detection of the change.

DE 39 31 828 A1 describes a method and an apparatus for reading a barcode which consists of a multiplicity of adjacent stripes made offerromagnetic material. A high-frequency electromagnetic alternatingfield is generated above the bar code so that a change of theelectromagnetic alternating field is caused by the ferromagneticstripes. By means of sensor coils which induce a changing voltage inaccordance with the changes, inductive recognition of the bar code ispossible. The induced voltage can be disturbed by external influences,however, so that the measured changes are distorted. To eliminate suchdisturbances from the measured signal, the measured signal isadditionally supplied to a synchronous demodulator and a low-passfilter. The measured signal is multiplied in the synchronous demodulatorby a reference signal of the same frequency and if possible the samephase. High-frequency components are subsequently filtered out in thelow-pass filter to obtain an adjusted signal containing substantiallyonly the measured changes. This type of signal processing is sometimesalso referred to as the lock-in principle.

A disadvantage of the above-mentioned inductive methods is that smallchanges of the electromagnetic alternating field, for example if only avery low concentration of ferromagnetic material is provided in thestripes or the exciting magnetic field is weak, are very difficult orimpossible to recognize. The primary reason for this is that there arefrequently interference fields in the measuring site that aresuperimposed on the measurement in such a way that an additional slightchange of the electromagnetic alternating field by a document to bemeasured is no longer reliably detectable by conventional means.

The problem of the invention is to specify a solution permittingreliable classification even of documents having small amounts ofmagnetic particles.

This problem is solved by the features of the independent claims.Advantageous embodiments and developments of the invention are stated independent claims.

According to the invention, a document to be checked containing magneticparticles is brought into an electromagnetic alternating field, thechange of the alternating field being measured using a measuring elementwhich converts an electrical input signal of the measuring element intoan electrical output signal in dependence on the electromagneticalternating field applied to the measuring element. Compared with thepurely inductive measurement procedures known from the prior art,measurement with such a measuring element has the advantage of beingtime-independent, since the change of electrical resistance in a giventest document depends only and directly on the strength of the appliedmagnetic field. In contrast, purely inductive methods are (also)time-dependent since a voltage is induced in the coil only when themagnetic flux penetrating the coil is subject to a temporal or spatialchange. The invention therefore permits a static measurement or ameasurement at slow document feed, thereby making the measurement moreexact.

Preferably, a measuring element is used wherein the electricalresistance of the measuring element changes in dependence on the changesof the electromagnetic alternating field. The measuring element can forexample be supplied with a current so that an alternating voltage dropsacross the measuring element. When the electrical resistance of themeasuring element changes due to the change of the alternating field,the amplitude of the applied alternating voltage also changes. Thisdetected amplitude change can then be processed further accordingly.

Particularly preferably, the measuring element used is amagnetoresistive element. Preferably, this is a giant magnetoresistance(GMR) element. In GMR elements the change of an external magnetic fieldapplied to the element causes a change in its electrical resistance.This enables the GMR element to convert magnetically coded informationinto an electrical signal by the amplitude of the output signal of theGMR element changing in dependence on the resistance value of the GMRelement. It is a special advantage of such a measuring element that evensmall changes of the electromagnetic alternating field can beascertained, since GMR elements have the property of changing theirelectrical resistance comparatively strongly upon even small magneticfield changes. Thus, GMR elements have increased sensitivity compared toother measuring elements or sensors. It is therefore possible to detecteven weakly doped documents that cause only a small field change. Due tothe increased sensitivity, authentic bank notes can furthermore bebetter distinguished from forgeries whose magnetic particle contentdiffers only slightly from that of authentic bank notes. Also, theinventive apparatus can be used variably and has little influence onadjacent systems, since the GMR sensor can also work at accordingly lowfield strengths due to its high sensitivity.

If the signal generator generates a high-frequency signal, a furtheradvantage of the GMR element can be utilized. The disturbing 1/f noiseof the GMR element occurs in a GMR element only in the low-frequencyrange and disappears above a certain frequency, leaving only a lower,white noise component. In this way a substantially highersignal-to-noise ratio is obtained. “High-frequency” means in thisconnection a frequency of more than 1 kHz, preferably over 10 kHz. Intests, magnetically soft and hard particles were detected with theinventive apparatus at a reference frequency of 7 kHz. Due to theproperties of the GMR element an improvement in the measurement resultsis to be expected at a reference frequency between 10 kHz and 50 kHz.The structure of GMR elements and their operation are described indetail for example in EP 0 793 808 B1.

It is provided according to the invention to process the output signalof the measuring element by means of a lock-in amplifier. If themeasuring element used is for example a GMR element, even very smallchanges of the electromagnetic alternating field can be detected by theGMR element. Since these changes result in comparatively small changesof the output signal of the GMR element, the output signal can beamplified in the lock-in amplifier for further processing. For thispurpose the GMR signal, possibly after being amplified, is multiplied bya normalized reference signal of the same frequency in a synchronousdemodulator. The signal generator with which the electrical input signalfor the GMR element is generated is preferably also used for generatingthe reference signal. Since the frequency of the GMR output signalalways corresponds to that of the GMR input signal independently of anyeffect of a magnetic field, the common signal generator can be used togenerate same-frequency signals for the lock-in amplification. To makesure that the reference signal is multiplied by the GMR output signal inphase, it is possible to use for example a phase-locked loop (PLL) forphase-locked regeneration of the reference signal.

The output signal of the synchronous demodulator subsequently passes alow-pass filter. The low-pass filter with a certain cutoff frequencyremoves the disturbing high-frequency components. After the low-passfilter has filtered out the high-frequency components, the resultobtained is an adjusted signal which is proportional to the amplitude ofthe GMR output signal.

Since the electrical output signal of the measuring element ismultiplied by a system-inherent reference signal of the same frequencyand phase, the reference signal used preferably being the input signalfor the measuring element, even small changes of the electromagneticalternating field which can be detected and verified by the measuringelement can be processed with high precision. An additional evaluationelectronics can be used to evaluate the measurements accordingly. Inparticular upon comparison measurements, the signal measured andprocessed by the lock-in amplifier must be compared with a given signaland/or other measured signals and evaluated. This comparison and theevaluation are then effected in the evaluation electronics, whereby theevaluation electronics can for example already comprise the lock-inamplifier.

The inventive apparatus can be used particularly advantageously formeasuring or recognizing magnetically soft particles in documents. Themagnetically soft particles are continually reversed magnetically by theelectromagnetic alternating field. The particles bundle the magneticfield lines, thereby strengthening the magnetic field. An advantage ofmagnetically soft materials is that they are readily magnetizable andcan therefore also strengthen weak magnetic fields. On the other hand,magnetically soft materials only slightly change the electromagneticalternating field, unlike magnetically hard materials, and thus provideonly a weak signal to be measured. With conventional measuring devicesthey are therefore not always reliably detectable. The invention alsopermits such materials to be reliably detected in documents, inparticular when a GMR element and/or a lock-in amplifier is used.

In one embodiment of the invention, the electromagnetic alternatingfield is generated by high-frequency bursts of a burst generator. Burstexcitation is understood to be the intermittent, bursty transmission ofa signal. Burst excitation permits a particularly high current load onthe field generating coil due to the lower average dissipation rate. Theaverage dissipation rate is lower in burst excitation since there is nopower dissipation in the burst pauses. If bursts with high currentintensity are used, the magnetic particles of the document aremagnetized accordingly more strongly and consequently cause a strongerchange of the alternating field. This causes an accordingly strongerchange in the electrical output quantity of the measuring element, i.e.its electrical resistance in the case of a GMR element, thereby makingthe measurement of the magnetic properties of the document more exact.

The inventive apparatus also makes it possible to distinguishmagnetically hard and soft particles. Magnetically hard materials have aconsiderably “broader” hysteresis loop than magnetically soft materials.That is, magnetically hard materials have a higher remanence, so thatconsiderably higher coercive field strength must be applied incomparison with magnetically soft materials to make this remanencedisappear. Consequently, magnetically hard materials have a higherremanence in the absence of an external magnetic field, i.e. in theabsence of current on the exciting coil, which manifests itself uponmeasurement with the measuring element in a greater change of theelectrical resistance of the measuring element. Due to these differentproperties of magnetically hard and soft materials, it can beascertained what kind of material is involved by a comparison ofdifferent measurements. For example, the particles can be premagnetizedin a premagnetization section. Measurements can then be carried out attimes when the coil does not generate an electromagnetic alternatingfield. It is particularly preferable to use burst excitation as theexcitation for the exciting coil, since in the no-pulse periods betweenthe recurring pulse bursts when the exciting coil does not carrycurrent, the materials to be measured are premagnetized and can bemeasured.

A measuring head for measuring changes of a magnetic field which can beadvantageously used with the present invention comprises at least oneexciting coil for generating a magnetic field and a giantmagnetoresistance (GMR) element for measuring changes of the magneticfield, the at least one exciting coil and the GMR element being disposedon a printed circuit board. The integration of the coil and the GMRelement on a printed circuit board is inexpensive and is thereforeadvantageous compared to known measuring heads. An evaluationelectronics suitable for evaluating changes of the magnetic field canalso be disposed inexpensively on the printed circuit board. Theevaluation electronics can comprise the lock-in amplifier.

The printed circuit board is preferably disposed between two elements,for example made of ferrite material, which concentrate the flux of themagnetic field generated by the at least one exciting coil. Furthermore,the arrangement of the exciting coil on the printed circuit board isexecuted inexpensively as a multilayer printed coil.

A further advantage of the inventive measuring head is the space-savingstructure. This makes it possible for example to dispose a multiplicityof measuring heads side by side in the above-described inventiveapparatus to permit measurements to be carried out at the same timealong a multiplicity of measuring tracks across the whole width of adocument to be examined.

According to a further idea of the present invention, themagnetoresistive elements used are not GMR elements but alternativelyso-called “spin-dependent tunneling” (SDT) elements. Said SDT elementshave a higher sensitivity than GMR elements by a factor of 10-20 and aretherefore particularly preferable.

It should be emphasized that the features of the dependent claims and ofthe embodiments stated in the following description, in combination orindependently of each other and in particular of the subject matter ofthe main claims, e.g. in magnetic measurement procedures without the useof lock-in amplifiers, describe further basic ideas and can beadvantageously used.

Further features and advantages of the invention will result from thefollowing description of various inventive embodiments and alternativesin connection with the accompanying drawings. These show:

FIG. 1 a measuring head;

FIG. 2 a schematic view of an inventive apparatus with the measuringhead from FIG. 1; and

FIG. 3 a preferred embodiment of a measuring head in cross section.

FIG. 1 shows a measuring head 1 for use in an inventive apparatus. Onecoil 3 is disposed on each of two parallel coil cores 2 which areconnected to each other at one end. When the coils 3 are supplied withcurrent they generate a magnetic field. An alternating current is usedhere so that an electromagnetic alternating field forms on the measuringhead 1. The coil cores 2 are connected to each other only at one end sothat an air-gap 4 forms between the free ends of the coil cores 2. Amagnetic stray field thereby forms at the free ends of the coil cores 2.A document 5 with magnetically soft particles, for example a bank notewhose ink in a printed image is provided with magnetically softparticles, is moved past the air-gap 4 of the inventive apparatus suchthat the stray field of the electromagnetic alternating field acts onthe magnetically soft particles. The flux density in the stray fieldarea is thus increased by the magnetically soft particles in thedocument 5.

A measuring element 6 which detects a corresponding change of theelectromagnetic alternating field is provided between the coils 3. Themeasuring element 6 can be for example a GMR element which changes itselectrical properties when a magnetic field is applied. The GMR elementis subject to a signal whose amplitude changes according to the changeof the magnetic field. The further processing of the amplitude-modulatedGMR output signal will be described hereinafter with reference to FIG.2. The GMR element is preferably disposed such that the magnetic field,which is generated for example by burst excitation of the two coils 3,is disposed perpendicular to the sensitive axis of the GMR element. Thisavoids overdriving of the GMR element. The GMR element is insensitive tomagnetic fields perpendicular to its principal sensitivity axis.

As mentioned above, a “spin-dependent tunneling” (SDT) element can alsopreferably be used, due to the higher measuring sensitivity, in additionor as an alternative to the GMR elements in this and all otherembodiments.

If such a measuring head 1 is to detect the total width of the testobject, the magnetic field must be dimensioned accordingly stronglybecause of the large air-gap. For example, an excitation burst of highcurrent intensity can be passed through the coils for this purpose.However, it is also possible to dispose a plurality of small measuringheads side by side.

FIG. 2 shows schematically an embodiment of the inventive apparatus. Inaddition to the measuring head 1 from FIG. 1, a lock-in amplifier 7 withat least some of its elements is shown. The output signal of the GMRelement is first pre-amplified by means of an amplifier 8 in the shownembodiment of the lock-in amplifier 7. The preamplified signal issubsequently supplied to a synchronous demodulator 9 whose operation hasbeen described above. The synchronous demodulator 9 is furthermoresupplied a reference signal which must have the same frequency as thepre-amplified signal in order for the lock-in amplifier 7 to provide thedesired result. Said reference signal is generated in a referencegenerator 10 and is furthermore used in the shown embodiment for theelectrical supply of the measuring head 1, in particular the GMR element6 with an input signal and the two coils 3 with an exciting signal.

Since conventional lock-in amplifiers are suitable only for processinganalog signals, the reference signal can be for example an alternatingcurrent signal. The changes of the electromagnetic alternating fieldwhich change the electrical resistance of the GMR element 6 cause achange in the amplitude of the voltage dropping across the GMR element6. This alternating current signal with changing amplitude is madeavailable to the lock-in amplifier 7 as the output signal of the GMRelement. After the preamplified GMR output signal has been multiplied bythe same-frequency reference signal in the synchronous demodulator 9, alow-pass filter 11 filters out spurious high-frequency components of thesignal, so that a signal is obtained that is proportional to the signalamplitude of the GMR output signal.

FIG. 3 shows an embodiment of a preferred measuring head 12 in crosssection. Multilayer printed coils 14 for generating an electromagneticalternating field and a giant magnetoresistance element 6 for measuringchanges of the alternating field are disposed on a printed circuit board13. The printed circuit board 13 itself is disposed between two elements15 which concentrate the flux of the magnetic field generated by thecoils in the plane of the GMR element 6. Said two elements 15 are madefor example of a ferrite material which is also suitable for use inconventional coil cores. As shown in FIG. 3, the document is movedtransversely to the vertically disposed measuring head 12.

1. A method for measuring magnetic properties of a document comprisingthe following steps: generating an electromagnetic alternating field,bringing a document into the electromagnetic alternating field,detecting changes of the electromagnetic alternating field while thedocument is located in the electrical alternating field, by means of ameasuring element which converts an electrical input signal into anelectrical output signal in dependence on the electromagneticalternating field, and processing the output signal by means of alock-in amplifier.
 2. The method according to claim 1, including using ameasuring element wherein the electrical resistance of the measuringelement changes in dependence on the changes of the electromagneticalternating field.
 3. The method according to claim 1, wherein, themeasuring element used is at least one of a giant magnetoresistance(GMR) element and a spin-dependent tunneling (SDT) element.
 4. Themethod according to claim 1, including using the input signal of themeasuring element as a reference signal for the lock-in amplifier. 5.The method according to claim 1, including using the input signal of themeasuring element as an excitation signal for the electromagneticalternating field.
 6. The method according to claim 1, including using ahigh-frequency signal is used as an input signal of the measuringelement.
 7. The method according to claim 6, wherein the high-frequencysignal used is a signal with a frequency of more than 1 kHz.
 8. Themethod according to claim 1, including using a burst signal as an inputsignal of the measuring element.
 9. An apparatus for measuring magneticproperties of a document comprising a device adapted to generate anelectromagnetic alternating field, a measuring element disposed in theelectromagnetic alternating field for measuring changes of theelectromagnetic alternating field, and a lock-in amplifier arranged toprocess an output signal of the measuring element, wherein the measuringelement is adapted to convert an electrical input signal of themeasuring element into the electrical output signal in dependence on theelectromagnetic alternating field.
 10. The apparatus according to claim9, wherein the device for generating the electromagnetic alternatingfield and the measuring element are integrated in a measuring head. 11.The apparatus according to claim 9, wherein, a signal generator isadapted to supply the measuring element with the input signal and thelock-in amplifier with a reference signal.
 12. The apparatus accordingto claim 11, wherein the signal generator is adapted to supply a signalfor generating the electromagnetic alternating field.
 13. The apparatusaccording to claim 12, wherein the reference signal and the signal forgenerating the electromagnetic alternating field are identical.
 14. Theapparatus according to claim 11, wherein the signal generator is adaptedto generate a high-frequency signal.
 15. The apparatus according toclaim 14, wherein the signal generator is adapted to generate a signalwith a frequency of more than 1 kHz.
 16. The apparatus according toclaim 11, wherein the signal generator is a burst generator.
 17. Theapparatus according to claim 11, wherein the signal generator isintegrated in the lock-in amplifier.
 18. The apparatus according toclaim 9, wherein the measuring element is at least one of a giantmagnetoresistance (GMR) element and a spin-dependent tunneling (SDT)element.
 19. The apparatus according to claim 9, wherein the device forgenerating the electromagnetic alternating field comprises two parallelcoil cores which are connected with each other at one end, at least onecoil being disposed on each of the two parallel coil cores.
 20. Ameasuring head for measuring changes of a magnetic field, comprising: atleast one exciting coil arranged to generate a magnetic field, and atleast one of a giant magnetoresistance (GMR) element and aspin-dependent tunneling (SDT) element arranged to measure changes ofthe magnetic field, wherein the at least one exciting coil (14) and thegiant magnetoresistance (GMR) or spin-dependent tunneling (SDT) elementare disposed on a printed circuit board.
 21. The measuring headaccording to claim 20, wherein the printed circuit board is disposedbetween two elements which are adapted to concentrate the flux of themagnetic field.
 22. The measuring head according to claim 20, whereinthe at least one exciting coil is disposed on the printed circuit boardin the form of a multilayer printed coil.
 23. The measuring headaccording to claim 20, wherein an evaluation electronics adapted toevaluate output signals of the GMR or SDT element is disposed on theprinted circuit board.
 24. The measuring head according to claim 23,wherein the evaluation electronics comprises a lock-in amplifier.